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Vascular Loops and Unilateral Hearing Loss

October 16, 2012

Dear Tucson Audiology Patients and Colleagues,

Last week we wrote about the benefits of induction loops for hearing.  Today, we’re writing about a different kind of loop — one you may have but perhaps wish you didn’t.  

Several weeks ago, we wrote about an unusual finding in a patient we’d referred for MRI to rule out acoustic neuroma–a benign, small, slow-growing tumor on the VIII cranial nerve–technically called a vestibular schwannoma.  The good news was that the MRI found no evidence of a tumor.  The surprise news was MRI identification of a “vascular loop” occupying some of the space reserved for the VIII nerve to emerge from the inner ear and enter the brain.  As we noted in that post, vascular loops can mimic symptoms of acoustic neuroma and that’s the topic of today’s post.  

http://wikis.lib.ncsu.edu/index.php/Nerve_and_blood_supply_of_the_inner_ear

Figure 1.  Anterior Inferior Cerebellar Arterial blood supply to audiovestibular system

The Story on Vascular Loops

Vascular loops are anatomical anomalies of the anterior inferior cerebellar artery (AICA).  We mention that artery because it is the blood supply for the inner ear — that includes hearing and vestibular (balance) systems.  Figure 1 gives you more information than you need but focus on the red, which is the blood supply.  Notice how it “shares” a narrow passage way with the vestibular and acoustic branches of the VIII cranial nerve.  Without the VIII nerve, we don’t hear.  Without the blood supply, same result.

The Bottleneck

Back to the sharing part:  in Figure 1, the artery and two nerve branches appear wrapped in a little cylinder.  That’s just a visual so you can see how they lie inside the skull. In actuality, all three pass through a short (about 1 mm) narrow little corridor (about 3.5 mm) that nature drilled into the temporal bone before entering the brain proper in what’s called the cerebellopontine angle (CPA).   The connecting corridor — called the internal auditory meatus — protects the blood and nerve supplies while linking the ear and vestibular system to the brain.   That doesn’t leave much room for error or extras. Speaking of extras, the facial nerve also goes through that corridor, though it’s not shown in figure 1.  Talk about a traffic jam!  No room left at all now.

The Traffic Jam: Injures But No Fatalities

Yet, into this bottleneck extra vascular AICA anomalies in the CPA can nudge and impose, “suspected of causing hearing loss, tinnitus, and vertigo” due to “the complex interaction between the vascular loop and eighth cranial nerve, in which the loop exerts pressure on the nerve, and the nerve compromises inner ear circulation.”

One study looked at 15 patients with unilateral (one-sided) or asymmetrical hearing loss and/or tinnitus who–like our patient–had imaging of the CPA suspected acoustic neuroma but were found to have vascular loops instead.  Complete audiovestibular work-ups on those patients revealed:

  • the already-established hearing loss on the side with the vascular loop
  • good, symmetrical, word recognition performance in both ears
  • spontaneous nystagmus (a vestibular sign) in 14 of the 15 subjects
  • a low proportion (one-third) of subjects with abnormal calorics (a test of the vestibular system)
  • no significant relationship between presence of tinnitus and presence or position of vascular loops vis-a-vis the auditory portion of the VIII nerve

The auditory findings of this study agreed with previous research by McDermott et al who reported a “clear” relationship between vascular loops and unilateral hearing loss, but not with tinnitus, in a prospective imaging study of over 300 patients with complaints of unilateral hearing loss and/or other “auditory symptoms.”

From these studies and others, Applebaum and Valvasorri arrived at the follow recommendation for practitioners:

Eighth nerve tumors and vascular loops produce similar symptoms, but a cochlear type of hearing loss with good speech discrimination and normal caloric testing should raise suspicion of a vascular loop.

According to the reports in the literature, as well as the recommendations our patient received from the neuro-otologist, surgery is rarely recommended for AICA loops.  The good news is it’s not a tumor.  The bad news is the hearing loss is here to stay.

Note for Audiologists:  Despite the suggested profile of “good, symmetrical word recognition scores,” our patient had poor speech discrimination on the involved side and almost 20% roll-over.  Even more reason to get that MRI!

Presented for your reading consideration by

Your Tucson Audiologists,

Holly Hosford-Dunn PhD

and

Sharon K Hopkins MA

References:  

Applebaum & Valvasorri.  Internal auditory canal vascular loops:  audiometric and vestibular system findings.  Am J Otol. 1985 Nov;Suppl:110-3.

McDermott et al.  Anterior inferior cerebellar artery syndrome: fact or fiction.  Clinical Otolaryngology & Allied Sciences. Volume 28, Issue 2, pages 75–80, April 2003.

photo courtesy of North Carolina State University

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